Certain materials are electroluminescent—that is, they emit light, and so glow, when an electric field is generated across them. The first known electroluminescent materials were inorganic particulate substances such as zinc sulphide, while more recently-found electroluminescent materials include a number of small-molecule organic emitters known as organic light emitting diodes (OLEDs) and some plastics—synthetic organic polymeric substances—known as light-emitting polymers (LEPs). Inorganic particulates, in a doped and encapsulated form, are still in use, particularly when mixed into a binder and applied to a substrate surface as a relatively thick layer; LEPs can be used both as particulate materials in a binder matrix or, with some advantages, on their own as a relatively thin continuous film.
This electroluminescent effect has been used in the construction of displays. In some types of displays an electroluminescent (EL) material—generally referred to in this context as a phosphor—is provided to form a backlight which can be seen through a mask that defines whatever characters the display is to show. In other types, the displays are arranged with shaped electrodes such that small individual areas of EL material can be selectively activated. These displays have many applications; examples are a simple digital time and date display (to be used in a watch or clock), a mobile phone display, the control panel of a household device (such as a dishwasher or washing machine), and a handheld remote controller (for a television, video or DVD player, a digibox, stereo or music centre or similar entertainment device).
International Patent Application No: WO 2005/0121878 describes a type of display that is an electroluminescent/liquid crystal hybrid. In such a display, an electroluminescent backlight is combined with a Liquid Crystal (LC) mask that is switchable in individual areas, between “on”/transparent (so that the backlight can shine therethrough) and “off”/opaque (so that the backlight's light is blocked thereby). The LC mask and the EL backlight are formed as a single integral unit wherein both components (the LC mask and EL backlight) are supported on a single substrate—the EL backlight being created as an electroluminescent material layer mounted directly on/behind the layer of liquid crystal material.
It has been found that in conditions where there is high intensity light incident on the display, such as in bright daylight, the weakly emissive EL backlight is difficult to make out as the light emitted by the EL backlight is overwhelmed by the brighter incident light reflected from the EL backlight. As a result, instead of a coloured icon, the display appears pale and colourless. Different phosphor colours become indistinguishable, all appearing pale, giving a “washed out” look to the display.
Attempts have been made to overcome this problem by ink jet coating or printing a transparent colour filter layer in front of the EL backlight and the LC mask such that undesirable wavelengths of incident light are absorbed. This ensures that the icon remains the desired colour. However, the filter layer affects the electric field across the EL and PDLC layers, adversely affecting performance, and the display is expensive to manufacture, requiring an additional print pass for each colour filter layer (typically one for each phosphor colour).